Ara-AC, a drug synthesized in this laboratory, has begun clinical trials in three institutions. Cyclopentenyl cytosine (CPE-C) has excellent preclinical activity against NCI in vivo tumor models as well as viruses in vitro. CPE-C is active against the standard L1210 leukemia tumor in mice and produces "cures" in the ara-C-resistant L1210 model. It also shows excellent activity against solid human tumor xenograft models (A549 lung, MX-1 mammary, metastatic LOX melanoma) in athymic mice. The compound has outstanding antiviral activity against DNA viruses (HSV-1 (TK+ and TK-), HSV-2, vaccinia, cytomegalovirus, varicella- zoster) and RNA viruses (influenza A2, vesicular stomatitis virus, Japanese encephalitis, Punta Toro, Rift Valley Fever) although, in several instances the antiviral therapeutic indices are low. Based on its antitumor activity, CPE-C has been selected as a potential clinical candidate by the DTP Decision Network Committee.